This mushroom has more than 17,000 sexes

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SSome common fungi likely have more than 17,000 sexes, researchers report March 31 at PLOS genetics. The work could help us better understand the evolution of sexual reproduction and show the growing power of genome sequencing.

While scientists have long suspected that some species of fungi have thousands, if not tens of thousands, of biological sexes, the new research has used cutting-edge genetic tools to confirm the extreme diversity of sexes in Trichaptum mushrooms.

“The rate of progress in DNA sequencing is simply breathtaking,” says Joseph Heitman, a geneticist and infectious disease expert at Duke University, who was not involved in the study but has previously studied DNA strategies. fungal mating. “This type of study would have been prohibitive in terms of cost and time even five years ago.”

Two compatible strains of Trichaptum fuscoviolaceum mating on petri dish

Vilde Bruhn Kinneberg

Trichaptum is a group of plate-like woody fungi (plateau fungi) that typically grow on trees and fallen logs in the colder parts of the northern hemisphere. “I think they’re quite pretty, but not very showy,” says Inger Skrede, a fungal biologist at the University of Oslo and co-author of the paper. The mushrooms weren’t chosen for their looks, of course. The international team of researchers decided to examine the species of this genus because they had already been studied by scientists and were easy to cultivate in the laboratory.

In total, the scientists obtained 180 specimens from three Trichaptum species; some they collected themselves, while others were sent to them by colleagues around the world (“You can just send it by regular mail,” says Skrede). From each fungal strain, they harvested spores, which were grown individually on agar plates for several weeks so that their DNA could be analyzed and they could be matched with growing spores from other specimens to see if they crossed each other.

Previous work had suggested that the sex of these fungi was controlled by two regions of the genome, known as MATA and MATB, and that each of these regions had many different possible alleles. For potential partners to be compatible, the two regions must be different from those of their potential partner.

This diversity has hampered sequencing efforts. The many divergent alleles make primer design virtually impossible, thwarting the use of less expensive targeted sequencing methods. This hurdle means that researchers wishing to sequence these fungi would have to rely on so-called next-generation short-read genomic sequencing technologies – methods which, given the number of individuals and the depth of sequencing needed to guarantee the accuracy, have simply been too expensive.

See “Fungi squeezed through microchannels offer clues to cell growth”

Now, however, the falling costs of next-generation technologies have made it possible to sequence so many individuals. And Skrede and his team were able to use new long-read sequencing to generate high-quality genomes that acted as scaffolds for precise assemblies of the short reads. The combination of methods allowed the researchers to determine precisely which parts of the MATA and MATB genetic regions were important for sex determination, as well as to count the number of different relevant variations in these two domains. Putting it all together, the team discovered that contained within these unassuming shelf mushrooms there could be a staggering 17,550 different combinations to choose from.

Microscopic image of attached hyphae

Coupling in Trichaptum occurs where the hyphae of two growing spores meet, as seen in this microscopic image of T. abietinum. The small bumps seen on the side of some hyphae are called pinches, which only occur after successful mating.

Dabao Sun Lu

Why an organism would need so much sexual variation remains an open question, but study author and University of Oslo geneticist David Peris suspects it has to do with the sessile lifestyle of organisms. mushrooms: being different in two different genetic regions makes it less likely that spores released from the same mushroom will successfully combine, thus reducing the chances of inbreeding.

Plus, having so many variants to choose from makes it more likely that given neighbors are sexually compatible — up to a 98% chance, Peris says — which could help the species survive in the long run. “When conditions change, you want to generate that genetic variation,” says Peris, because it can serve as fodder for adaptation.

Duur Aanen, an evolutionary biologist at Wageningen University in the Netherlands, says the study is such a good example of balanced selection – a phenomenon where the evolutionary favor of rare alleles leads to higher levels of ‘heterozygosity – that he plans to use it in his population genetics course.

However, Aanen, who was not involved in the study, also noted that there is one big question that none of the scientists who have examined this system have been able to fully answer: while diversity can help, why take it to this extreme? “If you have 100 alleles, you already have a lot of compatibility,” says Aanen. “What is the benefit of number 101?”

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